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GLAZES AND LUSTRES (Pottery, for artists)
CHAPTER XII GLAZES AND LUSTRES "I began to think that if I should discover how to make enamels I could make earthen vessels and other things very prettily, because God had gifted me with some knowledge of drawing." PALISSY. IT is in this department of potting, with its sur- prises, difficulties, and disappointments, its rare but exciting successes, that for most potters the greatest interest lies. To those of a scientific bent it is per- haps the summit of the craft, but the artist groping amidst formulas and methods may take heart. The finest work in pottery was not produced by scientists alone and does not depend altogether upon the quality of its paste, its unique colour, or strange lustre. The last word, the form, decoration, and craftsmanship, is with the artist. It is beyond the scope of a book of this description to enter into a necessarily long and complicated account of the different processes concerned in the composition of glazes. It will be sufficient to indi- cate their leading characteristics plainly enough to enable the student to start experimenting. None 117 118 can afford to miss opportunities for experiment, just as surely as none can afford to be always ex- perimenting. For a full description of materials and glazes and their manufacture such books as those of Drs. Shaw, Furnival, and Hainbach are recommended. Putting aside salt glaze we find a countless number of both lead and leadless glazes. They range from the thin silicious coating of the ancients up to the rich alkaline glazes of the Persians and Chinese; from the raw galena of peasant pottery to the rich Majoli- cas and fine hard glazes of modern commerce. Salt glaze is obtained by the vaporizing of salt inside the kiln at a great heat. The sodium oxide so formed combines with the silicates in the clay to form a very thin coat of refractory glass, intimately connected with the body. Porcelain glazes, though not differing so much in composition from the or- dinary fine earthenware glazes, are extremely hard, being compounded of kaolin, felspar, and quartz, with possibly limestone and ground sherds. It has in common with the salt glaze the close union with the body, so that when fractured the line of demarcation between glaze and body is indeter- minable. With a few minor exceptions the following list comprises the ingredients of all colourless glazes: 119 Kaolin Barytes White lead Quartz Bismuth Red lead Cornish stone Gypsum Zinc oxide Felspar Limestone Tin oxide Fluorspar Nitre Salt Flint Borax Soda Sand Bone ash Potash These materials must be free from all trace of iron. They are pulverized and some are calcined or oxi- dized. Then they are mixed in varying quantities to form the glaze mass. This mass is easily fusible when lead or borax is present in large proportions, more infusible or harder the more silica it contains, and very refractory if alumina is present in any quantity. The silica forming the glassy part of the glaze is stiffened by the presence of alumina, which stops any tendency to run. Lead is very largely used as a powerful flux at low temperatures but is unsuited to hard glazes. Borax and boracic acid are important constituents of leadless glazes. They are used to replace some of the silica, than which they are more fusible. Matt or non-reflective glazes are opaque and less vitreous than the glassy glazes. They do not flux or run. All these minerals are finely ground before mixing. Then those insoluble are mixed and fritted ; that is, fused in a crucible or fritting furnace to a greater or less degree, according to the hardness of the glaze. If 120 fused into a glass, the melted mass is poured into water to facilitate the next process, which is its re- duction to a fine powder by re-grinding. Then the completing ingredients are added and the mass coloured by the addition of metallic oxides. Of these the chief are : Iron Nickel Cobalt Manganese Copper Antimony Chrome Titanium and the more precious metals, in various forms, as oxides, carbonates, sulphates, and nitrates soluble in the glaze at great heat. These metals impart the many varied colours found in pottery. Zinc oxide is used to brighten a glaze or to stabilize colour. Tin oxide, which is insoluble at great heat but remains in suspension, gives opacity. This is no more than a skeleton outline of the in- tricate processes often used in the fabrication of a glaze. The manifold minerals, metals, oxides, acids, and alkalies are used in a variety of ways by the modern chemical potter. To all this seeming complication is added the ques- tion of pastes and bodies. There then arises the great problem of fitting one to the other. Salt glaze and porcelain excepted, the finished pot presents three stratas. Outside is the glaze, next the body, then inside the glaze again. If the coefficient of expansion of these three layers differs, in other words, if the glaze does not fit, the result is crazing, that bugbear of the potter. This crazing, which has been followed up and de- veloped into their delightful crackle by the Chinese, may show itself at once or only after a lapse of months. It appears as a minute network of fine cracks over the entire surface of the pot. It is often not unsightly, but sooner or later it must cause devitrification. The glaze after a time assumes an evanescent iridescence followed by a dull smoky appearance; finally, perhaps not for many years, it decomposes and peels off. With low-firing natural clays rich in silica and iron, the craze is not of much consequence. The body it- self at a moderately high temperature becomes non- porous. With hard short bodies containing lime or chalk it may have quite disastrous consequences. Water placed inside will eventually percolate through leaving a network of grey lines all over the pot and completely spoiling its appearance. It will be readily granted that, whether porous or non-porous, a craze is most undesirable on any piece of pottery that may be used for food or drink. It is here that the commercial potter is such an admirable fellow. Many of the glazes on modern tablewares are perfect for their purposes. Some- times only a little more fire is needed to stop the nuisance, but a bad craze usually needs more atten- 122 tion. The glaze requires stiffening, and the addition of ground flint or quartz, China stone or clay and felspar introduces alumina and silica and raises the fusing point. The substitution of borax for a portion of the silica can also be tried and will permit the use of slightly lower firing point. If the glazes are bought ready mixed, the body must be altered instead. Refractory China clays should be replaced by more fusible clays or some reduction made in the amount of infusible materials. The addition of ground sherds or flint will have a contrary effect should the glaze peel or crack at the edges, as it may do on a very silicious body. In working with natural clays on a moderate scale it will be found best to mix "fat" or rich natural clays with those of a more porous or hard nature. A few graded mixtures submitted to a thorough trial should soon show when a sympathy has been established between the body and glaze. The receipts given on pages 183 and 184 will make good colourless glazes without fritting if thoroughly ground in a mortar and passed through a sieve. Num- bers 3 and 7, when calcined, will give much more even results and they can be coloured by the addition of the oxides named. But simple as it sounds, the washing, grinding, fritting, re-grinding, and sieving is a long and laborious process demanding machinery, and on that account is unsuited to schools or potters of mod- 123 erate means. Glazes like Nos. 1 and 2 will do quite well for elementary work but unless the appliances are to hand the manufactured article will have to be relied on for more finished and ambitious work. If, indeed, you are already in possession of a good receipt for a fine colour and glaze, one quite worth while, so much the better. Mix it and feel the joy of the Compleat Potter unafraid of spoiling his own good shapes with a faulty or unknown glaze. Admittedly, to get anywhere in an original direc- tion systematic research is essential. One must keep on experimenting, keep on hoping, and keep on taking notes ; but at the start let us not be too impatient or independent if we wish to produce good pots. There is often among young potters a false pride that prevents them using, and among old potters acknowledging the use of, the manufactured article. Why this should be is a little difficult to understand. A painter might far more reasonably be ashamed to use modern tube colours or a stained-glass craftsman as logically insist on making his own glass, as a potter in the twentieth century refuse to avail him- self of the wonderful range of glazes that modern research has placed at his disposal. These resources should be used intelligently, not mechanically, or by the book artistically, inventively, secretly, if you will, but they should be used until the multitudi- nous experiments have borne fruit and repeated trials 124 convince you that at last you possess some gem of research worth, as well it may be, the months of patient toil engendered in its production. The various receipts are given on pages 183 and 184 without analysis of the composition of the paste or body to which they were applied. The first group have been used on common earthenware clays with complete satisfaction. They are to be considered as points of departure for future experiments in which they may be modified at will, and not regarded as a contribution to the science of glaze making. In colouring it will be found that combinations of cobalt, iron, and copper oxides give an interesting range of simple blues or greens ; iron and manganese browns ; and so forth. The colour mass or stain is ground fine and lawned, and from about 2 to 7 per cent mixed with the colourless glaze mass, according to the depth of colour required. The ordinary under-glaze colours may be used to stain glazes, the percentage being fixed by small trials. For the rare colours turquoise, crimson, or purple a more complicated process is necessary and only perfected after many trials. The ingredients of these fine colours are naturally kept secret by their fortunate possessors. It must be noted that a glaze suited to one body may peel or run off an unsuitable one. Then a colourant is affected differently by a lead or 125 an alkaline base in the glaze. Again, copper and iron oxides may help to flux a glaze, whilst cobalt or nickel will exert a contrary effect. Cobalt, being a strong colourant, will need a sparing use, whereas a similar percentage of iron will merely tinge the glaze mass. And so ad infinitum. It is self-evident that any attempt to emulate the vast range of the modern ceramic chemist is doomed to failure. To a craftsman the fabrication of one fine individual glaze or lustre is an achievement of which he may be proud, and for which he will find abundant and varied uses. In this connection it is encouraging to the craftsman to learn from so high an authority as W. Burton, Esq., F.C.S., that it is impossible to obtain with purified oxides the fine tones got by the Orientals with impure materials. Further, that the simple glaze of the Persians a mixture of clean white sand with soda or wood ash or potash is still the best for under-glaze painting. Although tastes differ so widely, invariably it will be found that more and more heat will be the cry. Imperceptibly this leads to the desire for hard, cold, "fat" translucent glazes, neither matt nor glossy. And on the summit, far out of reach, stand the wonders of the Old Chinese. 126 LUSTRES There are several kinds of lustres, but the true lustres possess a pearly iridescence in addition to their colour. The copper and silver lustres of the eighteenth-century Staffordshire potters were thin metallic films over the whole surface of the glaze. Gold or silver solutions were used. Only where the gold was fired on a white clay is there any iridescence, and then hardly so pronounced as to deserve the name of lustre. The bismuth and other lustres made by the modem potter are combinations of metallic oxides and resinates dissolved in ethereal oils. These are painted on the glaze, transparent or opaque, but having almost invariably a lead base, and then fired at a dull-red heat. The medium disappears, and the metal in a finely divided state is deposited on the surface of the glaze. This, however, gives a painted look very different from the lovely irradiance of the Persian Hispano-Mooresque or Italian work. With these the lustres were fired in a reducing at- mosphere, one supercharged with combustible gases, the metals decomposed and fixed to the semifluxed glaze. The manipulation was not infallible and was attended with much risk ; but the successful pieces are unrivalled, and according to Piccolpasso "paid in gold." De Morgan, who more nearly approached the works of the Italian masters than any other 127 modern potter, used a very similar medium and method with his fine lustres. In the last few years lustres have been brought to such perfec- tion, in preparation, application, and firing, that accident is practically eliminated. Glorious colours and gorgeous effects are obtained. Yet it may be said without senseless adulation of the merely medi- aeval that nothing has been produced superior, or even akin in spirit, to the work sent out from Persia, Spain, or the botega of the inimitable Maestro Georgio of Gubbio. Lustres may be bought ready to apply. They are then painted on the glazed pot, which is re-fired at a dull-red heat in the ordinary oxidizing atmos- phere. Once the painted look has been contrasted with the lustrous appearance of the reduced effects there is only one kind of lustre for the artist. For- tunately, perhaps, their preparation is not easy, and the correct method of reducing is a difficulty to be overcome by practical experiment alone. Hainbach gives many practical receipts for lustres that are not beyond the scope of a craftsman. The reducing at- mosphere can be obtained in an open kiln by the in- troduction at the right moment of any combustible giving smoke free from all traces of sulphur. In firing with a muffle kiln the introduction of coal-gas free from sulphur is a matter at- tended with some risk at the necessarily low tern- 128 peratures required by the lustres. It should be approached with caution, and each mixture will have a varied firing point, the correctness of which is established only by trials. Lustres applied with skill and restraint enhance the most beautiful glaze, but in unskilled hands they inevitably vulgarize and cheapen. The fresh, unsophisticated renderings of the Persians or the Moors and the virility of the Italians should be studied, but not merely imitated before working in this medium. ^ PAINTING OUTHT FIG. 55 Categoría:Pottery, for artists, craftsmen & teachers Categoría:lustre